I'm not sure the apt style WH is the correct solution for OP
homeowner.

That said Ransley numbers seem credible to me, the unit he describes
can deliver about 4 gpm of water from 55F to 125F.

It's like the best of both worlds; 90gal storage & 4 gpm

I've got a house with a 75 gal WH (about 75,000btu/hr I think ) serves
3 full baths & you definitely don't want to be the 4th or 5th shower
(esp if those early ones were"longish")

It also serves a 75 gallon spa tub that it cannot fill with premixed
water; you've got to start out with all hot & as themp falls it just
turns out right.

It's all about the BTU's per minute demanded & the available supply;
you've got to store, be able to generate it or reduce instaneous
demand.

Existing gas line will determine what capacity is available.

I've toyed with OP's Option #4 but the cost of tankless has detered me.
My idea would be set the temperature of the tank heater above the
temperature of the tankless. The tankless would come on only when
needed.

This kinda defeats the purpose of the tankless but the problem with
tankless is when current demand exceeds capacity you no longer get "hot
water" you get "warmed water".

Most tankless can only supply 2 major users at the same;

two showers
shower & bath
shower & washer
etc

as soon as you add the 3rd user on the tankless, performance will
suffer

Maybe tank + tankless makes sense; I've haven't been sure enough (or
cold showered enough) to do anything about

The Takagi T-KJr. Tankless Water Heater is a lower capacity (140,000
btu/hr max) & lower priced unit that might make sense.

cheers
Bob